With good single-target damage, regular AoE freezes, wide ranging, and a strong ultimate, on his own, Boreas is rather good. He needs the support of freeze units such as Marri or Ai, however, to make him really stand out. With the extra freezing from others, his anti-healing and bonus damage stats start to show through, making him great as a DPS and control mage.  While not god-tier on his own, Boreas is one of the better mages, and when well-supported, becomes great.